Variable-speed constant-potential generator



Nov.' 2, 192e. 1,605,294 Y G. H. SETHMAN VARIABLE SPEED CONSTANT POTENTIAL GENERATOR A Filed 'August 24; 1925 Y r /z v 'george yeman.

batte ry that the ignition system of `the engine.

f battery Patented Nov. 2, 1926.

UNITED STATES PATENT orifice.

GnRGR HL sETHMAN, or DENVER, COLORADO.

VARIABLE-Seann cOivsTANr-POTNTL GENERATOR.

rIhis invention relates to improvements in variable speed constant Vpotential gener'- ators of a type that finds its principal use in connect-ion With the electrical starting and ignition systems of automobiles.

In automobile construction the generator is geared directly to the engine and there- Ytore its speed varies directly with thespeed of the engine. It is obvious, therefore', that this type of generator must be so constructed when the generatorfhas attained awvo'ltage for the starting `motor,

` of' volts. After the pick-up yoltage has which is usually afttafiried when the automobile reaches speed approXimatly ten miles an hour, the yolta'ge of the generater should remain practically constantv for, any speed at which it maybe operated. This regulation h as` hee1i attained in present constructions means of a third rush that `is placed approximately at right angles' to the main coinrnutat'or brushes and which supplies a shunt fielclyvindingl As the speed ofthe generatorincreasesthe field becomes distorted and this, together With been attained,

the cross magnetization vof the v arrrratur` e, field excitation to' such an tends to vary the extent that the voltage generated does njot exceed a reasonable amount regardless of the speed of' the generator.

In this type-of generator1 however, for any reason Vthe cut-'ontniechanism should fail' to Work and therefore no` current sheuld be delivered by Vthe generator to'jthe battery,

the' me magnetization. ne armature.

would be rednc'edV pr"cticallyy to z'eroi v vhile the' increased voltage-would send an increased current through the result that the vltage cpnstfantly increases Withthe speed untllth'e shunt field bnrns' out, dueto thisexcess'iveyoltagq l1: is the. Object off this mentent@ nioduce4 a generator iff tyhich the shunt, tela 4with the voltage' will never exceed a predetermined value, regardless of Whether the circuit Ifrom the armature to the battery is open or closed. My invention brieflly described consists in constructing a generator in such a rvay that the pole pieces are formed very soft iron and are fastened to v a .Y sotliat there isno complete magnetic circuit in the generally accepted sense. To thesepole pieces so connected to each other, sectire a permanent electro-magnet that magnetitesA the pole pieces and proouces finit, of a certain value, between the adjacent pieces and through the armature. The armature Winding` is then calculated sov that when thearmature rotates at the speed that cdrresponfds to a speed of approximate- 13" voltage generated'-between the brushes is 7 volts, or suficient to operate the cut-out mechanism and connect the generator to the battey.

base of vnon-magnetic metalten niiles an.f hour'A of the automobile, the

It is evident that if' I depended `merely i loii the permanent field that I would be unattain the vr egnlation Which I desire, as the field Would approximately cons 'tant'uwhereasthe distortion and the crossyrnafgn'etizati'on would vary ,with the speed ajhld'load. I To compensate for this variation I provided my generator with brushes placed approximately 90 degrees apart, one, of which is'l adjustable so as to 'increase the angle Y.to any extent that may be found necessary,`y in order to produce `the proper regulat"ion.l The conductors from lone of these brushes entend to a series field Winding of feyv'V turns so that the vcurrent generated by the machine will all he employed to affect the field' strength ,of the generator, The number of tiirns inthe series field is lso calculated that l it Will counterbalance the effect produced by the field distortion' and the cross-magnetization of the armature for any loadsl between zero and maximum load, and at any ep'eed.

Iirorder to more clearly describe my invention I shall have referencetothe accompanying drawing'in which it has been illustrated diagrammatically and in which Fic?. l shoivs in a dia lram'niatic iva the C t l EGO .latter will receive a charge.

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'with the commutator at diametrically opposed points and lie in a plane substantially perpendicular to the plane o1 the pole pieces. A third brush 8 contacts with the armature at some point between brushes G and 7, and is adjustable so as to increase the angle between it and brush 6 from 90 to approximately 180 degrees. Brush 8 is connected to the series iield coils 9 and the battery l() is connected between the conductor 11 that connects with the iield coil and the Vconductor l2 that connects with brush 6.

In Fig. 2 I have indicated by means of lines 13 the flux that passes between the poles as a result otl the magnetizing action of the permanent magnet. Then the generator 1s .rotated in the direction indicated by the ar row in Fig. l, the armature conductors will cut the lines offorce and set up an electromotive force between the brushes in a manner well understood. The armature windr ing has been so calculated that when the generator attains a speed corresponding to a speed of ten miles an hour by the auto.

mobile the pick-up voltage will be reached. This voltage as above indicated isY usually about 7 volts. 'Vhen this voltage is attained, the cutout 14 will automatically connect the generator to the battery and the As the speed of the automobile increases, if other conditions remained constant, the voltage would also increase, which would increase the charging' current to the battery. This, however, is prevented by the fact that as the speed increases there will be a corresponding increase in the distortion ot the iield, which will cause the lines of iiorce to pass from one pole to the other in a diagonal direction, somewhat as shown in Fig. 3. This distortion weakens the field between brushes 6 and 8 with the result that even at the increased speed only the same voltage will be generated. eIn addition to the distortion there is a cross-magnetizing eii'ect produced by the armature, which however remains substantially constant for constant currents. By properly proportioning the number of turns in the lield winding 9, it is possible to produce a magnetizing effect that is just sufficient to maintain the voltage between brushes 6 and 8 constant for any speeds at which the generator may be working. This is due to the fact that the magnetic circuit is substantially open, as the permanent magH net by which the pole pieces are connected is saturated or nearly so and therefore it has no appreciable effect on the flux produced by the series winding.

It is evident that it the brush 8 is moved upwardly to one or the other ot the dotted line positions shown in F ig. l, that a greater this tendsto produce constant voltage for any load, while the eld distortion due to the increased speed compensates itor the voltage increase that would otherwise arise due to the increased speed.

By properly adjusting the brush S and vproperly calculating the effect of the coil 9,

it lis evident that constant voltage can be attained at any load and'at any speed.

, Let usassume a case in which the cut-out fails to operate and in which the automobiles speed is gradually increased to a very high rate. As longl as there is no current flowing through the series coil this is of course ineiiective and the only flux that affects the armature is that produced by the permanent magnet. ever, distorts this ield into such a position that very little of this iield affects the coils between brushes 6 and 8, with the result that the voltage remains practically constant or rises very little above the calculated maximum. As the held winding is so related to the magnetic circuit that it has an accu1nulative effect on the magnetization due to the permanent magnet, it is obvi ous that it tends to strengthen the permanent magnet at all times, so that the latter always remains fully charged. This assures that there will always be a constant field that produces the pick-up voltage at substantially the same speed in all cases. Since the pole pieces l and 2 are quite massive and are formed of metal. having a very small retentivity, the iiuX is easily displaced and the regulation for speed is therefore readily obtained.

I also wish to call particular attention to the fact that the magnetic circuit through the permanent magnet is for all practical purposes ineffective so tar as it may have relation to the carrying of additional fiux that is produced by the series coil, for the reason that the permanent magnet is made of very hard steel and is practically saturated rlhe high speed, how-l at all times. Besides this, the magnetic circuit through the permanent magnet is so small that its effect is practically nil. There is therefore only a leakage path for the flux produced by the coil 9, as it cannot return through the permanent magnet.

It Will appear from the above that I have produced a generator' that is exceedingly Well adapted for use in connection with automobiles, as it has been so designed that it produces practically constant voltage at all speeds and automatically regulates itself, With respect to the load as Well as to the speed; that this effect has been produced by exceedingly simple means which make it possible to construct this generator at very small cost, By means lof this generator A there is absolutely no danger of overcharging the battery or of burning out the generator. Even Where the cut-out :tails to Work and the speed is excessive, the voltage Will increase only slightly above the normal, and even if the voltage did increase to a considerable extent there Would be no danger of burning out the generator, due to the fact that there are no shunt windings.

Having novv described my invention What I claim as new is:

A constant voltage variable speed generator comprising in combination, a permanent magnet, a soft iron pole piece of small retentivity attached to each pole of saidv electromagnet, said pole pieces being provided with concave faces adapted to cooperate with an armature, an armature mounted for rotation between said poles, a commutator associated with said armature, a pair of brushes operatively associated With said commutator, one of said brushes being located substantially at the neutral point of said commutator and the other brush contacting with the commutatorv at4 a point spaced from the iirst named brush an angle of less than 180 degrees, in the direction ot rotation, and a series lield coil connected to one of said brushes.

In testimony whereof I affix my signature.

GEORGE H. SETI-IMAN. 

